Rewrite email parser using ImapEngine, harden processing loop

Replace webklex/php-imap with directorytree/imapengine in the ticket
email parser. ImapEngine is pure PHP over sockets.

Parser improvements:
- Wrap per-message processing in try/catch so one malformed email
  can't abort the run; failures are flagged and logged with UID
- Query unseen + unflagged so previously-failed (flagged) messages
  are no longer re-processed on every cron run
- Skip vacation/auto-responder emails (RFC 3834) to prevent mail
  loops with the ticket auto-reply
- Cap messages per run (50) and attachment size (15MB); inline
  images over 2MB are stored as attachments instead of base64-embedded
  in ticket details
- Atomic lock file creation
- preg_quote() the ticket prefix in subject matching
- Dedupe CC watchers and exclude the sender
- Map webklex 'tls' encryption setting to STARTTLS for compatibility

NDR/DSN parsing now walks MIME parts via the underlying
zbateson parser instead of relying on attachment extraction.

plugins/vendor/php-di/invoker/src/InvokerInterface.php

@@ -0,0 +1,25 @@
+<?php declare(strict_types=1);
+
+namespace Invoker;
+
+use Invoker\Exception\InvocationException;
+use Invoker\Exception\NotCallableException;
+use Invoker\Exception\NotEnoughParametersException;
+
+/**
+ * Invoke a callable.
+ */
+interface InvokerInterface
+{
+    /**
+     * Call the given function using the given parameters.
+     *
+     * @param callable|array|string $callable Function to call.
+     * @param array $parameters Parameters to use.
+     * @return mixed Result of the function.
+     * @throws InvocationException Base exception class for all the sub-exceptions below.
+     * @throws NotCallableException
+     * @throws NotEnoughParametersException
+     */
+    public function call($callable, array $parameters = []);
+}